Waterjet systems (e.g., abrasive jet systems) are used in precision cutting, shaping, carving, reaming, and other material-processing applications. During operation, waterjet systems typically direct a high-velocity jet of fluid (e.g., water) toward a workpiece to rapidly erode portions of the workpiece. Abrasive material can be added to the fluid to increase the rate of erosion. When compared to other material-processing systems (e.g., grinding systems, plasma-cutting systems, etc.) waterjet systems can have significant advantages. For example, waterjet systems often produce relatively fine and clean cuts, typically without heat-affected zones around the cuts. Waterjet systems also tend to be highly versatile with respect to the material type of the workpiece. The range of materials that can be processed using waterjet systems includes very soft materials (e.g., rubber, foam, leather, and paper) as well as very hard materials (e.g., stone, ceramic, and hardened metal). Furthermore, in many cases, waterjet systems are capable of executing demanding material-processing operations while generating little or no dust, smoke, and/or other potentially toxic byproducts.
In a typical waterjet system, a pump pressurizes fluid to a high pressure (e.g., 40,000 psi to 100,000 psi or more). Some of this pressurized fluid is routed through a cutting head that includes an orifice element having an orifice. Passing through the orifice converts static pressure of the fluid into kinetic energy, which causes the fluid to exit the cutting head as a jet at high velocity (e.g., up to 2,500 feet-per-second or more) and impact a workpiece. The orifice element can be a hard jewel (e.g., a synthetic sapphire, ruby, or diamond) held in a suitable mount (e.g., a metal plate). In many cases, a jig supports the workpiece. The jig, the cutting head, or both can be movable under computer and/or robotic control such that complex processing instructions can be executed automatically.
Certain materials, such as composite materials and brittle materials, may be difficult to process using conventional waterjet systems. For example, when a waterjet is directed toward a workpiece made of a composite material, the waterjet may initially form a cavity in the workpiece and hydrostatic pressure from the waterjet may act on sidewalls of the cavity. This can cause weaker parts of the workpiece to preferentially erode. In the case of layered composite materials, for example, hydrostatic pressure from a waterjet may erode binders between layers within the workpiece and thereby cause the layers to separate. As another example, when a waterjet is directed toward a workpiece made of a brittle material (e.g., glass), the load on the workpiece during piercing may cause the workpiece to spall and/or crack. Similarly, spalling, cracking and/or other damage can occur when waterjets are used to form particularly delicate structures in both brittle and non-brittle materials. Other properties of waterjets may be similarly problematic with respect to certain materials and/or operations.
One conventional technique for mitigating collateral damage to a workpiece (e.g., a workpiece made of a composite and/or brittle material) includes piercing the workpiece with a waterjet at a relatively low pressure (e.g., corresponding to a relatively low pressure upstream from an orifice) and then either maintaining the low pressure during the remainder of the processing or ramping the pressure upward after piercing the workpiece. At relatively low waterjet pressures, waterjet processing is often too slow to be an economically viable option for large-scale manufacturing. Furthermore, conventional techniques for ramping waterjet pressures upward (e.g., by ramping fluid pressure upstream from an orifice upward) can also be slow and typically decrease the operational life of at least some components of waterjet systems. For example, a conventional technique for ramping waterjet pressures upward includes controlling a pump and/or a relief valve to increase the pressure of all of the pressurized fluid within a waterjet system. With this technique, a variety of components of the system (e.g., valves, seals, conduits, etc.) are repeatedly exposed to the fluid at both low and high pressures. Over time, this pressure cycling can lead to fatigue-related structural damage to the components, which can cause the components to fail prematurely. Greater numbers of pressure cycles and greater pressure ranges within each cycle tend to exacerbate these negative effects. The costs associated with such wear (e.g., frequent part replacements, other types of maintenance, and system downtime) can make such approaches impractical for certain applications. For example, in material-processing applications that involve repeatedly starting and stopping a waterjet (e.g., to cut spaced-apart openings in a workpiece), ramping system pressures in each instance can cause unacceptable wear to conventional waterjet systems and make use of such systems for these applications cost prohibitive.